Looks like an opportunity for adapter makers to offer adapters with a built-in anti-reflective (both sides) filter in them.

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A7 with kit lens and a number of legacy lenses (mostly Canon FD)

The adapters' supposed internal reflections have been worked to death with gaffer, sugru and stuff: no change .

The A7 reflections look like a diffracted and enlarged reflection of sensor's pixel pattern, either reflected back to the rear lens and then again onto the sensor, or the effect of angled rays being furterly diffracted by sensor glass cover.